Generally, the Venn Diagram method is OK for assessing validity.
But, to sort of
supplement that method for examining validity we can add the following rules.
First, we should remind ourselves of distribution of terms.
Remember the following:
Statement type
Term(s) distributed
A
subject
E
subject, predicate
I
nothing
O
predicate
Rule 1:
The middle term must be distributed at least once.
Fallacy:
undistributed middle
Example:
All sharks are fish.
All salmon are fish.
All salmon are sharks.
(you need not know the following per se; it only explains the reason for the rule)
The point is the following: (assuming M denotes the middle term, S the minor term, and
P the major term) if M is distributed in the first premise, then P will be related to ALL of
the M class.
At that point, if M is related to S, then there will be some common ground
from which to draw.
Similarly, if M is ever distributed in the second premise, then S will
be related to the whole class of M.
Then, if M is related to P at all, we’ll know something
about the relation between all three.
If M is undistributed throughout, then all we know is
that S is somehow related to M, and P is somehow related to M, not that S is in any way
related to P, which will kind of be VITAL for the conclusion.
Rule 2: If a term is distributed in the conclusion, then it must be distributed in a
premise.
Fallacies:
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 Spring '08
 Barrett
 Conclusion, premises, tigers, Traditional logic, Syllogistic fallacy

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